Systems and methods for beamforming in a massive MIMO system

ABSTRACT

Embodiments provide systems and methods for enabling a first transceiver to learn beamforming weights (e.g., Eigen beamforming weights) to a second transceiver, without any pilot signaling or explicit beamforming weight signaling from the second transceiver. In another embodiment, beamforming weight vectors to enable a multi-symbol spatial rate can be learned by the first transceiver.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of U.S. ProvisionalApplication No. 61/812,029, filed Apr. 15, 2013, which is incorporatedherein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates generally to antenna systems and methodsfor Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) communication.

2. Background Art

In a Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) communicationsystem, a transmitter, such as a base station, is equipped with a verylarge number of transmit antennas (e.g., 32, 62, or 100) that can beused simultaneously for transmission to a receiver, such as a userequipment (UE). The receiver can have more than one receive antenna(e.g., 2, 4, 8, etc.) or even a very large number of receive antennasfor simultaneously receiving transmissions from the transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the pertinent art to makeand use the disclosure.

FIG. 1 illustrates an example environment in which embodiments can bepracticed or implemented.

FIG. 2 illustrates an example process according to an embodiment.

FIG. 3 illustrates an example transceiver according to an embodiment.

FIG. 4 illustrates another example process according to an embodiment.

FIG. 5 illustrates an example mapping of logical antenna ports tophysical antennas.

FIG. 6 illustrates an example logical antenna port pilot transmissionconfiguration according to an embodiment.

FIG. 7 illustrates an example process according to an embodiment.

The present disclosure will be described with reference to theaccompanying drawings. Generally, the drawing in which an element firstappears is typically indicated by the leftmost digit(s) in thecorresponding reference number.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of this discussion, the term “module” shall be understoodto include at least one of software, firmware, and hardware (such as oneor more circuits, microchips, processors, or devices, or any combinationthereof), and any combination thereof. In addition, it will beunderstood that each module can include one, or more than one, componentwithin an actual device, and each component that forms a part of thedescribed module can function either cooperatively or independently ofany other component forming a part of the module. Conversely, multiplemodules described herein can represent a single component within anactual device. Further, components within a module can be in a singledevice or distributed among multiple devices in a wired or wirelessmanner.

FIG. 1 illustrates an example environment 100 in which embodiments canbe practiced or implemented. Example environment 100 is provided for thepurpose of illustration only and is not limiting of embodiments. Asshown in FIG. 1, example environment 100 includes a first transceiver102 and a second transceiver 104. First transceiver 102 can be a basestation and second transceiver 104 can be a user equipment (UE), forexample, or vice versa.

As shown in FIG. 1, transceiver 102 includes M transmit antennas 110.1,. . . , 110.M, which provide transceiver 102 with a transmit/receivespace with M spatial dimensions. Transceiver 104 includes Ntransmit/receive antennas 112.1, . . . , 112N, resulting in an M×Nuplink channel H 106 from transceiver 104 to transceiver 102 and an N×Mdownlink channel H′ 108 from transceiver 102 to transceiver 104. N canbe equal to, lower than, or greater than M according to embodiments.

In order for transceiver 104 to transmit, and receive optimally to/fromtransceiver 102 over channels 106 and 108 respectively, transceiver 104needs to learn channels 106 and 108. Conventionally, transceiver 102transmits pilot symbols from each of its antennas 110.1, . . . , 110.M(e.g., for each antenna, pilots are transmitted over a number oftime/frequency resources that are orthogonal to the time/frequency/coderesources used by the other antennas) to transceiver 104. This enablestransceiver 104 to estimate downlink channel 108. In a Time DivisionDuplexing (TDD) system (i.e., where the same frequency resources areused for uplink and downlink transmission), assuming channel reciprocity(i.e., that the uplink and downlink channels are similar over the samefrequency resources), transceiver 104 can also infer the uplink channel106 from the estimate of downlink channel 108. Transceiver 104 can thenuse the knowledge of the uplink channel to beamform transmissions totransceiver 102.

In another conventional scheme, transceiver 102 estimates uplink channel106 using pilot symbols signaled by transceiver 104. Transceiver 102then signals the estimate of uplink channel 106 and/or uplinkbeamforming weights to transceiver 104. Transceiver 104 can infer thedownlink channel 108 from the estimate of uplink channel 106 and can usethe signaled uplink beamforming weights to beamform transmissions totransceiver 102.

However, these conventional schemes can be resource inefficient. Forexample, in the case of a large number of antennas 110.1, . . . , 110.Mat transceiver 102 (e.g., when transceiver 102 is a MassiveMulti-Input-Multi-Output (MIMO) (M-MIMO) base station with a largenumber of transmit/receive antennas (e.g., 100 transmit antennas)),pilot signaling from each antenna to transceiver 104 requires a verylarge overhead. Similarly, explicit signaling of beamforming weights canwaste resources, especially in the case of a multi-user system in whichtransceiver 102 can serve multiple transceivers 104.

Embodiments, as further described below, provide systems and methods forenabling transceiver 104 to learn uplink beamforming weights (e.g.,Eigen beamforming weights) for transmission to transceiver 102, withoutany pilot signaling or explicit beamforming weight signaling fromtransceiver 102. In another embodiment, beamforming weight vectors toenable a multi-symbol spatial rate can be learned by transceiver 104. Inanother aspect, schemes for reducing the amount of pilot signalingneeded from transceiver 102 to enable transceiver 104 to learn downlinkchannel 108 are provided.

FIG. 2 illustrates an example process 200 according to an embodiment.Example process 200 is provided for the purpose of illustration only andis not limiting of embodiments. For the purpose of illustration only,example process 200 is shown as being performed by example transceivers102 and 104 described above with reference to FIG. 1.

As shown in FIG. 2, example process 200 begins in step 202, whichincludes transceiver 104 applying channel sounding on all of its Nantennas to transceiver 102. In an embodiment, this includes, for eachof antennas 112.1, . . . , 112.N, transmitting a number of referencesignals (known to transceiver 102) over respective time/frequencyresources that are orthogonal to time/frequency resources used by theother antennas. In an embodiment, where transceiver 102 is a basestation and transceiver 104 is a UE, the plurality of reference signalscan be sounding reference signals (SRS), transmitted by the UE over aset of frequency tones reserved for channel sounding by the UE to thebase station. Antennas 110.1, . . . , 110.M of transceiver 102 receivethe reference signals from antennas 112.1, . . . , 112.N of transceiver104 simultaneously over respective orthogonal frequency tones, forexample, or at different times over same/different frequency tones.

Subsequently, in step 204, process 200 includes transceiver 102estimating the M×N uplink channel H 106 from transceiver 104 totransceiver 102 based on the channel sounding performed by transceiver202. Then, in step 206, process 200 includes transceiver 102 performinga singular value decomposition of the estimate of channel H 106. Thesingular value decomposition decomposes H 106 into a unitary matrix U, adiagonal matrix D, and a unitary matrix V. In an embodiment, the matrixD is sorted such that the first vector corresponds to thestrongest/optimal vector. The singular value decomposition of H 106 canbe described mathematically as:H _(M×N) =U _(M×M) D _(M×N) V _(M×N) ^(H)  (1)

Process 200 then proceeds to step 208, which includes transceiver 102transmitting to transceiver 104 a pilot symbol (known to transceiver104) beamformed by a vector u₁*, corresponding to the conjugate of thefirst column vector of the unitary matrix U. In an embodiment, wheretransceiver 102 is a base station and transceiver 104 is a UE, the pilotsymbol can be UE-specific Demodulation Reference Signals (DMRS) symbolas defined by the Long Term Evolution (LTE) standard. In an embodiment,the same beamformed DMRS symbol is transmitted simultaneously on thesame frequency tone by all M antennas of first transceiver 102.Effectively, the M antennas of first transceiver 102 appear as a singleantenna to second transceiver 104 for this pilot symbol transmission.

Subsequently, process 200 proceeds to step 210, which includestransceiver 104 estimating a beamformed downlink channel (i.e., theestimate includes the applied beamforming in step 208) based on thepilot symbol transmitted by transceiver 102. The pilot symbol isreceived by all N antennas of transceiver 104. In an embodiment, thebeamformed channel can be represented mathematically as:h ₁ ^(T) =H ^(T) u ₁ *=V*DU ^(T) u ₁ *=v ₁ *d ₁  (2)where v₁* corresponds to the first column vector of the unitary matrixV, and d₁ is a scalar. In an embodiment, v₁ is the optimal beamformingvector from transceiver 104 to transceiver 102, and thus by estimatingthe beamformed channel transceiver 104 can immediately determine theoptimal beamforming vector to transceiver 102. In an embodiment,transceiver 104 determines v₁ from the beamformed downlink channelestimate as

${v_{1} = \frac{h_{1}^{*}}{h_{1}}},$here h₁* is the conjugate of h₁ and ∥h₁∥ is the norm of h₁.

Process 200 terminates in step 212, which includes transceiver 104transmitting a signal to transceiver 102 using a beamforming vectorw=v₁, where w is an N×1 weight vector. In an embodiment, this isequivalent to performing maximum ratio transmission (MRT) beamformingfrom transceiver 104 to transceiver 102 over channel 106, whichmaximizes Signal-to-Noise Ratio (SNR) at the M antennas of transceiver102.

FIG. 3 illustrates an example transceiver 300 according to anembodiment. Example transceiver 300 is provided for the purpose ofillustration only and is not limiting of embodiments. Exampletransceiver 300 can be an embodiment of transceiver 102 described aboveand can be used to perform example process 200 with another transceiver(e.g., transceiver 104). As shown in FIG. 3, example transceiver 300includes a channel estimator 304, a singular value decomposition (SVD)module 308, a beamforming vector generator 312, a beamforming module330, an Inverse Fast Fourier Transform (IFFT) module 326, and aplurality of antennas 110.1, . . . , 110.M.

In an embodiment, antennas 110.1, . . . , 110.M are configured toreceive a plurality of first reference signals transmitted respectivelyfrom a plurality of antennas of another transceiver (e.g., antennas112.1, . . . , 112.N of transceiver 104). For example, each of theplurality of antennas of the other transceiver can transmit a number offirst reference signals over respective time/frequency resources thatare orthogonal to time/frequency resources used by the other antennas.Antennas 110.1, . . . , 110.M can receive the plurality of firstreference signals simultaneously over respective orthogonal frequencytones, for example, or at different times over same/different frequencytones. In an embodiment, where example transceiver 300 is a base stationand the other transceiver is a UE, the plurality of first referencesignals can be sounding reference signals (SRS), transmitted by the UEover a set of frequency tones reserved for channel sounding by the UE tothe base station.

Channel estimator 304 is configured to receive an input signal 302,which includes the plurality of first reference signals, and to estimatean uplink channel H 306 from the plurality of antennas of the othertransceiver to the plurality of antennas 110.1, . . . , 110.M. Where theother transceiver includes N antennas, the uplink channel H 306 isrepresented by an M×N matrix. Channel estimator 304 outputs uplinkchannel H 306 to decomposition module 308.

SVD module 308 is configured to perform a singular value decompositionon uplink channel H 306 to generate a singular value decomposition (SVD)310. Singular value decomposition 310 represents H 306 as the product ofan M×M unitary matrix U, an M×N diagonal matrix D, and an N×N unitarymatrix V*. Since V is unitary, its column vectors are orthogonal to eachother. The same applies for column vectors of U.

Beamforming vector generator 312 is configured to process SVD 310 togenerate a beamforming matrix 314. In an embodiment, beamforming matrix314 is formed from column vectors (each column vector is M×1) of theunitary matrix U. For example, beamforming matrix 314 can be representedas U*=[u₁*u₂* . . . u_(r)*], where u₁* corresponds to the conjugate ofthe first column vector of U, u₂* corresponds to the conjugate of thesecond column vector of U, and so on. Beamforming matrix 314 can have rcolumn vectors (r≦M), where r is determined by the spatial rate oftransmission (i.e., number of different streams transmittedsimultaneously over orthogonal frequencies) from example transceiver 300to the other transceiver.

Beamforming module 330 is configured to receive one or more secondreference signals 316.1, . . . , 316.r (s₁, . . . , s_(r)) and tomultiply the one or more second reference signals 316.1, . . . , 316.rby respective vectors 314.1, . . . , 314.r of beamforming matrix 314 togenerate one or more beamformed reference signals 320.1, . . . , 320.r.In an embodiment, beamforming module 330 includes a multiplier bank 318that comprises a plurality of multipliers 318.1, . . . , 318.r, eachconfigured to multiple (e.g., in the frequency domain) a respectivesecond reference signal 316.k with a respective vector 314.k ofbeamforming matrix 314 to generate a beamformed reference signal 320.k(1≦k≦r). In an embodiment, where example transceiver 300 is a basestation and the other transceiver is a UE, the one or more secondreference signals 316.1, . . . , 316.r can be UE-specific DemodulationReference Signals (DMRS) as defined by the Long Term Evolution (LTE)standard.

In an embodiment, beamformed reference signals 320.1, . . . , 320.r arecombined using a combiner 322 to generate a signal 324. Signal 324 canbe buffered and then acted upon by IFFT module 326 to generate amulti-carrier modulated signal 328, such as an Orthogonal FrequencyDivision Multiplexing (OFMD) symbol. In an embodiment, IFFT module 326include M IFFTs, one for each antenna 110.1, . . . , 110.M, and signal328 is a vector of size M. Each of the M elements of signal 328 isproduced by a respective one of the M IFFTs and is forwarded to arespective one of the plurality of antennas 110.1, . . . , 110.M fortransmission to the other transceiver. In an embodiment, the M elementsof signal 328 are transmitted simultaneously by their respectiveantennas 110.1, . . . , 110.M. As a result, each beamformed referencesignal 320 (e.g., 320.1) is transmitted simultaneously, on the samefrequency tone, by all of antennas 110.1, . . . , 110.M. Further, theone or more beamformed reference signals 320.1, . . . , 320.r aretransmitted simultaneously on orthogonal frequency resources to theother transceiver.

FIG. 4 illustrates an example process 400 according to an embodiment.Example process 400 is provided for the purpose of illustration only andis not limiting of embodiments. Example process 400 can be performed bya first transceiver having a plurality of first antennas, such asexample transceiver 300 described above, to enable a second transceiverhaving a plurality of second antennas to learn uplink beamformingweights (e.g., beamforming vector) to the first transceiver. The firsttransceiver can be a base station and the second transceiver can be aUE, or vice versa.

As shown in FIG. 4, process 400 begins in step 402, which includesreceiving, using the plurality of first antennas of the firsttransceiver, a plurality of first reference signals transmittedrespectively from the plurality of second antennas of the secondtransceiver. In an embodiment, where the first transceiver is a basestation and the second transceiver is a UE, the plurality of firstreference signals can be sounding reference signals (SRS), transmittedby the UE over a set of frequency tones reserved for channel sounding bythe UE to the base station.

Subsequently, process 400 proceeds to step 404, which includesestimating a channel from the plurality of second antennas to theplurality of first antennas based on the plurality of first referencesignals. In an embodiment, where the first transceiver includes Mantennas and the second transceiver includes N antennas, the channel canbe represented by an M×N matrix.

Then, in step 406, process 400 includes decomposing the channel into aunitary matrix U, a diagonal matrix D, and an unitary matrix V. In anembodiment, step 406 includes performing a singular value decompositionof the channel. In an embodiment, step 406 further includes forming abeamforming matrix having one or more vectors of the unitary matrix U.

Finally, process 400 terminates in step 408, which includestransmitting, from the plurality of first antennas, one or morebeamformed reference signals, the one or more beamformed referencesignals resulting from multiplying one or more second reference signalsby respective vectors of the unitary matrix U of the channel. In anembodiment, where the first transceiver is a base station and the secondtransceiver is a UE, the one or more second reference signals can beUE-specific Demodulation Reference Signals (DMRS) as defined by the LongTerm Evolution (LTE) standard.

In an embodiment, step 408 includes transmitting a first beamformedreference signal resulting from multiplying a respective one of the oneor more second reference signals by a first column vector of the unitarymatrix of the channel. In another embodiment, step 408 includestransmitting the first beamformed reference signal simultaneously byeach of the plurality of first antennas of the first transceiver. Asdescribed above with reference to FIG. 2, when the second transceiverestimates the channel based on the first beamformed reference signal (orany beamformed reference signal), the generated estimate will beproportional to (e.g., scaled version of) a beamforming vector that canbe used by the second transceiver to beamform transmissions to the firsttransceiver. In an embodiment, the beamforming vector enables maximumratio transmission (MRT) beamforming from the second transceiver to thefirst transceiver over the channel.

In another aspect, referring to FIG. 1, embodiments include schemes forreducing the amount of pilot signaling needed from transceiver 102 toenable transceiver 104 to learn downlink channel 108. Exampleembodiments of such schemes are now provided. Generally, these exampleembodiments are described with reference to the LTE standardarchitecture. However, embodiments are not limited to LTE as would beapparent to a person of skill in the art based on the teachings herein.

FIG. 5 illustrates an example mapping 500 of logical antenna ports tophysical antennas. Example mapping 500 is provided for the purpose ofillustration and is not limiting of embodiments. Example mapping 500 canbe used in a MIMO transceiver, such as an LTE-based base station(eNodeB), for example.

As shown in FIG. 5, example mapping 500 maps logical antenna ports 502to physical antennas 504. Each antenna port can be mapped to one or morephysical antennas. Transmitting on a logical antenna port includestransmitting using all of its mapped physical antennas. When a logicalantenna port is mapped to more than one physical antennas, both data andpilot signals are multiplied by respective weights for the multiplephysical antennas. For example, transmitting on logical antenna port AP5is done by transmitting using all of physical antennas PA0, PA1, PA2,and PA3 by multiplying the signal being transmitted by weights w_(5,0),w_(5,1), w_(5,2), and w_(5,3) for physical antennas PA0, PA1, PA2, andPA3 respectively.

Typically, a receiver at the receiving end of transmissions fromphysical antennas 504 (e.g., a UE) is aware of only logical antennaports 502 and their associated physical resource blocks (time andfrequency blocks). For example, the receiver may have knowledge thattransmissions from AP0 occur on specific frequencies during specifictime slots. However, the receiver does not know which of physicalantennas 504 is/are used to transmit from AP0. As a result, downlinkchannel estimation at the receiver is performed with respect to logicalantenna ports 502 as opposed to physical antennas 504. For a logicalantenna port that is mapped to a single physical antenna (e.g., AP0),the corresponding downlink channel encompasses the channel(s) from thephysical antenna (e.g., PA0) to the receiver antenna(s). For a logicalantenna port that is mapped to multiple physical antennas (e.g., AP5),the receiver estimates a composite channel from the multiple physicalantennas (e.g., PA0, PA1, PA2, and PA3) to the receiver antenna(s). Thecomposite channel includes the weights (e.g., w_(5,0), w_(5,1), w_(5,2),and w_(5,3)) associated with the physical antennas during transmission.

Conventionally, in order for the receiver to estimate the entiredownlink channel, the transceiver transmits pilot symbols for each ofits supported logical antenna ports (e.g., for each logical antennaport, pilot symbols are transmitted from its associated physicalantenna(s) over respective physical resource blocks). However, this canbe very resource inefficient. This is especially true in the case ofM-MIMO systems, where the number of logical antenna ports can be verylarge (e.g., 16, 32, 64, etc.).

Embodiments, as further described below, recognize that in MIMO andM-MIMO systems, physical antennas tend to be closely spaced to eachother (e.g., a grid). As a result, a spatial correlation typicallyexists between physical antennas as well as between transmissions fromlogical antenna ports. Embodiments exploit this characteristic to reducethe amount of pilot signaling needed to enable downlink channelestimation. Specifically, embodiments limit pilot signaling to only asubset of supported logical antenna ports and rely on spatialcorrelation information to interpolate channels from logic antenna portsfor which no pilot signaling is used.

FIG. 6 illustrates an example logical antenna port pilot transmissionconfiguration 600 according to an embodiment. Example configuration 600is provided for the purpose of illustration only and is not limiting ofembodiments. For the purpose of illustration only and not limitation, itis assumed that the transceiver (e.g., base station) using configuration600 supports 16 logical antenna ports, and that each of the 16 logicantenna ports is mapped to a single distinct physical antenna of anantenna grid with 16 physical antennas as shown in FIG. 6. However,embodiments are not limited to this example and can be extended to caseswhere a logical antenna port can be mapped to more than one physicalantenna.

In an embodiment, example configuration 600 includes transmitting pilotsymbols for only a subset of the supported logical antenna ports (pilottransmission subset). For example, pilot signaling can be performed foronly those logical antenna ports that are mapped to physical antennasillustrated using circles in FIG. 6. However, no pilot symbols aretransmitted for the logical antenna ports that are mapped to physicalantennas illustrated using squares in FIG. 6 (no-pilot transmissionsubset).

A receiver (e.g., a UE) at the receiving end of the pilot signaling cancalculate the respective channels for the pilot transmission subset oflogical antenna ports. For example, the UE can determine the channel forthe logical antenna port mapped to physical antenna 602. Then, withknowledge of example configuration 600 (e.g., knowledge of the pilottransmission subset) and spatial correlation information among thelogical antenna ports, the receiver can determine by interpolation therespective channels for the no-pilot transmission subset of logicalantenna ports. For example, the UE can interpolate from the channeldetermined from physical antenna 602 to determine the channel for thelogical antenna port mapped to physical antenna 604. Such interpolationcan take into account the relative positions of physical antennas 602and 604, accounting for any differences such relative positions can haveon transmissions to a receiver. In embodiment, these differences can bedetermined mathematically, by approximation, and and/or using actual apriori testing.

The transceiver can signal the logical antenna port pilot transmissionconfiguration and the spatial correlation information to the receiver.In another embodiment, the receiver can compute the spatial correlationinformation using Angle of Arrival (AoA) information associated withpilot signals received from the transceiver. The receiver can alsocompensate this computed spatial correlation information based onchanging environment conditions (e.g., fast changing channel or slowchanging channel). In an embodiment, the spatial correlation informationincludes a physical antenna spatial configuration, which can include oneof: actual positions of the plurality of physical antennas, relativepositions of the plurality of physical antennas (e.g., positions of thephysical antennas relative to a fixed coordinate point), a spatialcorrelation matrix of the plurality of physical antennas of thetransceiver (e.g., a co-variance matrix that provides a spatialcorrelation value between any two physical antennas), and a valueidentifying a physical antenna configuration (e.g., number and locationsof physical antennas) known to the receiver. In an embodiment, thespatial correlation information can also include a mapping of logicalantenna ports to physical antennas, such as example mapping 500, forexample. In another embodiment, the spatial correlation informationincludes a spatial correlation matrix of the logical antenna ports(e.g., which already accounts for the mapping of logical antenna portsto physical antennas).

FIG. 7 illustrates an example process 700 according to an embodiment.Example process 700 is provided for the purpose of illustration only andis not limiting of embodiments. Example process 700 can be performed bya receiver, such as a UE, to determine a channel from a transmitter,such as a base station, having a plurality of physical antennas andsupporting a plurality of logical antenna ports.

As shown in FIG. 7, process 700 begins in step 702, which includesdetermining a physical antenna spatial configuration of the transmitter.In an embodiment, step 702 further includes receiving from thetransmitter one of: actual positions of the plurality of physicalantennas of the transmitter, relative positions of the plurality ofphysical antennas of the transmitter, and a spatial correlation matrixof the plurality of physical antennas of the transmitter. In anotherembodiment, step 702 also includes receiving actual or relativepositions of the plurality of physical antennas of the transmitter; andcalculating the spatial correlation matrix of the plurality of physicalantennas using the actual or relative positions of the plurality ofphysical antennas. In a further embodiment, step 702 also includesreceiving a mapping of logical antenna ports to physical antennas.

Process 700 then proceeds to step 704, which includes determining alogical antenna port pilot transmission configuration of thetransmitter. In an embodiment, the logical antenna port pilottransmission configuration includes information regarding first logicalantenna ports of the plurality of logical antenna ports with pilotsignal transmission (pilot transmission subset) and second logicalantenna ports of the plurality of logic antenna ports without pilotsignal transmission (no-pilot transmission subset). In an embodiment,step 704 includes receiving the logical antenna port pilot transmissionconfiguration from the transmitter or retrieving a pre-determinedconfiguration from memory.

Subsequently, in step 706, process 700 includes receiving a pilot signalfrom one or more physical antennas of the plurality of physical antennasof the transmitter. Then, in step 708, process 700 includes estimating afirst channel for a first logical antenna port based on the receivedpilot signal. The first antenna belongs to the pilot transmission subsetand is mapped to the one or more physical antennas from which the pilotsignal is transmitted.

Process 700 terminates in step 710, which includes interpolating thefirst channel, using the physical antenna spatial configuration and thelogical antenna port pilot transmission configuration, to estimate asecond channel for a second logical antenna port of the plurality oflogical antenna ports. The second logical antenna port belongs to theno-pilot transmission subset of the plurality of logical antenna ports.

In another embodiment, process 700 can be performed using a logicalantenna port spatial configuration (e.g., a spatial correlation matrixof the logical antenna ports, which accounts for the mapping of logicalantenna ports to physical antennas) instead of a physical antennaspatial configuration.

Embodiments have been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of embodiments of the present disclosure shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method for conveying a beamforming vector from a first transceiver to a second transceiver, comprising: receiving, using a plurality of first antennas of the first transceiver, a plurality of first reference signals transmitted respectively from a plurality of second antennas of the second transceiver; estimating a channel from the plurality of second antennas to the plurality of first antennas based on the plurality of first reference signals; decomposing the channel into a first unitary matrix, a diagonal matrix, and a second unitary matrix; and transmitting, from the plurality of first antennas, one or more beamformed reference signals, the one or more beamformed reference signals resulting from multiplying one or more second reference signals by respective vectors of the first unitary matrix of the channel, wherein said transmitting comprises transmitting, from the plurality of first antennas, a first beamformed reference signal simultaneously on a same frequency by each of the plurality of first antennas, the first beamformed reference signal resulting from multiplying a respective one of the one or more second reference signals by a first column vector of the first unitary matrix of the channel.
 2. The method of claim 1, wherein the plurality of first reference signals include sounding reference signals.
 3. The method of claim 1, wherein decomposing the channel comprises performing a singular value decomposition of the channel.
 4. The method of claim 1, wherein an estimate of the channel based on the first beamformed reference signal at the second transceiver is proportional to the beamforming vector being conveyed from the first transceiver to the second transceiver.
 5. The method of claim 1, wherein the beamforming vector enables maximum ratio transmission (MRT) beamforming from the second transceiver to the first transceiver over the channel.
 6. The method of claim 1, wherein the one or more second reference signals include Demodulation Reference Signals (DMRS).
 7. The method of claim 1, wherein the first transceiver is a base station and the second transceiver is a user equipment (UE).
 8. A transceiver, comprising: a plurality of first antennas configured to receive a plurality of first reference signals transmitted respectively from a plurality of second antennas of another transceiver; a channel estimator configured to estimate a channel from the plurality of second antennas to the plurality of first antennas based on the plurality of first reference signals; a decomposition module configured to decompose the channel into a first unitary matrix, a diagonal matrix, and a second unitary matrix; and a beamforming module configured to multiply one or more second reference signals by respective vectors of the first unitary matrix of the channel to generate one or more beamformed reference signals, wherein the one or more beamformed reference signals include a first beamformed reference signal, the first beamformed reference signal resulting from multiplying a respective one of the one or more second reference signals by a first column vector of the first unitary matrix of the channel, and wherein the plurality of first antennas are further configured to simultaneously transmit on a same frequency the first beamformed reference signal to the other transceiver.
 9. The transceiver of claim 8, wherein the plurality of first reference signals include sounding reference signals.
 10. The transceiver of claim 8, wherein the decomposition module is further configured to perform a singular value decomposition of the channel.
 11. The transceiver of claim 8, wherein the one or more second reference signals include Demodulation Reference Signals (DMRS).
 12. The transceiver of claim 8, wherein the plurality of first antennas are further configured to transmit the one or more beamformed reference signals simultaneously to the other transceiver on orthogonal frequency resources.
 13. The transceiver of claim 8, wherein the transceiver is a base station and the other transceiver is a user equipment (UE).
 14. A base station, comprising: a plurality of first antennas; a decomposition module configured to decompose a channel from a plurality of second antennas of a user equipment (UE) to the plurality of first antennas into a first unitary matrix, a diagonal matrix, and a second unitary matrix; and a beamforming module configured to multiply one or more reference signals by respective vectors of the first unitary matrix of the channel to generate one or more beamformed reference signals, wherein the one or more beamformed reference signals include a first beamformed reference signal, the first beamformed reference signal resulting from multiplying a respective one of the one or more reference signals by a first column vector of the first unitary matrix of the channel, and wherein the plurality of first antennas are configured to simultaneously transmit on a same frequency the first beamformed reference signal to the UE.
 15. The base station of claim 14, wherein the plurality of first antennas are further configured to receive a plurality of first reference signals transmitted respectively from the plurality of second antennas of the UE.
 16. The base station of claim 15, further comprising: a channel estimator configured to estimate the channel based on the plurality of first reference signals.
 17. The base station of claim 14, wherein the plurality of first antennas are further configured to transmit the one or more beamformed reference signals to the UE.
 18. The base station of claim 15, wherein the plurality of first reference signals include sounding reference signals.
 19. The base station of claim 14, wherein the decomposition module is further configured to perform a singular value decomposition of the channel.
 20. The base station of claim 14, wherein the one or more reference signals include Demodulation Reference Signals (DMRS). 